Presses often utilize irregular screen geometries because there are many more irregular screens than regular ones. As a result, irregular screen geometries provide a larger set of multi-separation screen combinations for avoiding interaction between screening frequencies of concurrent screens. However, irregular screen geometries often have harmonic artifacts as described in U.S. patent application Ser. No. 10/844,993. That application for patent shows how the artifacts, which originate from the digital design may be reduced. There are, however, additional harmonic artifacts that are often generated due to process variations (the analog part of the electrophotographic (EP) printing process), when using irregular screen geometries, and to a lesser extent when using regular screens. One such process variation exists in screen designs that utilize partial exposures in order to increase the native resolution of a press, as discussed in U.S. patent application Ser. No. 10/950,841.
For example, a press with a native resolution of 800 dpi may be operated at a virtual resolution of 2400 dpi by utilizing two partial exposure levels, ⅓ and ⅔, in addition to the 0 and 1 levels. Preparation of a halftone image using a conventional clustered dot technique includes grouping output pixels to form a halftone dot that varies in size and proximity to other halftone dots to convey analog pixel intensity information, for instance, a shade of grey that cannot be conveyed by varying output pixel intensity. Output pixels generally have a binary intensity. For example, a pixel is black when printed or white when not printed.
The halftone image is typically prepared by a computer, laser printer, ink printer, copier, or facsimile machine by digital computation and then stored in whole or in part prior to transferring the image to a tangible medium, such as printable media, film, or a printing plate. Translation by digital computation usually introduces some quantization and possibly inaccurate variation of halftone dot size and placement. Variations in the halftone dot size and placement also arise due to process variations caused during the transfer of the image to the tangible medium. One result of these variations is that they may give rise to undesirable, visible patterns known generally as screening artifacts. Screening artifacts are not pleasing to the eye and often produce patterns that are not in the original object. For example, a large area of the same color in the original can have a pattern with objectionable variation in color in the reproduced image.
It would thus be desirable to be able to achieve the better dot positioning, while substantially reducing or eliminating the visibility of screening artifacts caused by variations in the halftone dot size and placement.